MeiosisMaking Sex Cells

Why Did Sex Evolve in the First Place?

• The simplest method of reproduction is asexual reproduction. Asexual reproduction results in offspring that are clones of the parent organism.

• Asexual reproduction is favored when the environment is relatively stable, because if the parent is well-adapted to the environment, their offspring will be well-adapted too.

Aww, how cute. It looks exactly like me…

Why Did Sex Evolve in the First Place?

• So why reproduce sexually? Sexual reproduction gives organisms the opportunity to combine their genetic information with that of another organism.

• As a result, their offspring typically have a greater variety of traits than those produced by asexual reproduction.

Why Did Sex Evolve in the First Place?

Asexual reproduction:

Parent: Large leaves ↓

Offspring: All large leaves

Why Did Sex Evolve in the First Place?

Sexual reproduction:

Parents: Large leaves + Small leaves ↓

Offspring: Some large, some medium, some small

How Are Sex Cells Made?• Sex cells, also known as gametes, are made

through the process of meiosis.

• Gametes are sperm and eggs.

• In humans, gametes are produced by the reproductive system.

What Is Meiosis?

• Meiosis is a 2-stage process by which the chromosome number in a cell is reduced by ½

• A somatic cell (a body cell) is called a diploid cell because it contains a full set of chromosomes.

• A gamete (a sex cell) is called a haploid cell because it contains a half set of chromosomes.

1. A somatic cell from a ring tailed lemur has 56 chromosomes. How many chromosomes will be in one of these lemur’s gametes? ______

2. Is this gamete haploid or diploid? _______

3. What would happen if gametes did not go through meiosis?

____________________________________________________________________

How Does Meiosis Increase Genetic Diversity?

• Meiosis increases the diversity of the traits in the offspring in 2 ways:

• Crossing over – During prophase I of meiosis, homologous chromosomes form a tetrad and exchange DNA

Crossing Over – Draw This

New combination of DNA

How Does Meiosis Increase Genetic Diversity?

• Independent assortment – homologous chromosomes line up independently – they can line up in any order

• So, the gametes are NOT ALL THE SAME

Independent Assortment

So How Do The Gametes Get Together?

• When gametes combine, it is called fertilization

• The resulting cell is called a zygote. Over time, the zygote will grow into an embryo.

So How Do The Gametes Get Together?

• External fertilization – many gametes are released into water by both sexes

• Internal fertilization – the male often has a copulatory organ (penis) for transferring sperm to the egg

External Fertilization

Internal Fertilization

How Does This Compare to Mitosis?

Mitosis Meiosis

Purpose

Number of cells made

Are cells genetically identical?

Are cells haploid or diploid?